A FND diagnosis doesn’t mean you are imagining things or making them up, so you are as entitled to see the doctor as anybody else.
Even if it is the FND (not unlikely, since that is what you are diagnosed with), that doesn’t mean you should not be getting any help with it.
FND is as valid a Dx as MS, so that’s like me saying I couldn’t go to the doctor’s, because they’ll say it’s all MS. They’d very likely be right, but that doesn’t mean I shouldn’t go, or tell them what’s been happening (not that I do often - probably not as much as I should, but that’s beside the point).
We don’t really understand FND, at the moment, and it may not actually have a single cause, but really be lots of different diseases. But that doesn’t mean there is no treatment that helps at all. Some of the drugs, such a neuropathic painkillers, commonly prescribed for MS might also help with FND. Are you on any of those? Have you been offered the chance to try?
It’s a bit of a vicious circle, because if you won’t go to the doctor, because you don’t believe they’ll do anything, you’ll never find out if you were wrong.
If your current doctor seems a bit dismissive, and claims nothing can be done, is it possible you could see anyone else at the same practice? Because reasons for the symptoms are not yet understood, treatment might be a bit more trial-and-error than for other conditions, but I think some GPs would at least be willing to try, so if yours isn’t, perhaps it’s time for a second opinion?
I’m not promising there’s some miracle cure out there, that’s currently being withheld. It might be that you try a few things that don’t work anyway. But I think some GPs will be more proactive than others about letting you have a go.